Developments in ultrasound technology have resulted in access to fascinating real-time three-dimensional images of the fetus and have enabled greater public and clinical insight into early fetal development and movement. Dr Julie-Claire Becher provides an overview of our latest understanding of the early stages of fetal development.
- Ultrasound provides remarkable pictures of the fetus allowing the study of early development as well as diagnosis of congenital abnormalities.
- Fetal movement begins about 7.5 weeks after conception and by 14 weeks flexion, extension, rotation, thumb sucking and yawning occur.
- Even when seemingly purposeful, early movements are due to reflexes occurring at spinal cord level.
- Purposive movement depends on brain maturation. This begins at about 18 weeks and progressively replaces reflex movements, which disappear by about 8 months after birth. Persistence of reflex activity is common when brain damage has occurred.
- The fetus can hear from around 23 weeks, and shows response to maternal speech. Fetal learning has been shown in response to sound.
- Most cerebral palsy is due to brain injury acquired in the womb. Ultrasound study of fetal behaviour may identify such abnormal neurological development before birth.
Declaration of interests: None declared.
The development of ultrasound technology has resulted in real-time three-dimensional (4D) images of the fetus that are remarkable to the public and physicians alike. Not only do such images offer more accurate delineation of congenital anomalies such as facial clefts, central nervous system abnormalities and cardiac defects but there are obvious benefits for intrauterine procedures such as fetal surgery. Obstetricians have also found parental reactions to the pictures of their unborn baby particularly rewarding.
Motor and sensory development
Images of the fetus ‘walking’ and ‘thumb-sucking’ have come as a surprise to the public, but such movements have, for several years, been well described in the literature. For those working in obstetrics and ultrasonography, these pictures are therefore remarkable only in their definition and real-time technology. There is no doubt that the fetus has complex movements, but what does it all actually mean?
Images showing the fetus taking steps are not surprising to paediatricians. Newborn infants can be shown to ‘walk’ when held upright on a flat surface. Even preterm infants can do so a short time after birth. Many of the behaviours demonstrated by the newborn in the early postnatal period, are a function of primitive reflex activity, dependent on the development of spinal reflex arcs which are not under control of the brain and are complete in the fetus as early as eight weeks’ gestation. Reflexes are very different from purposeful voluntary movements which develop during the first year of life. Such movements are dependent on the maturation of the central nervous system, and in particular on myelination, which starts from around 18 weeks’ gestation. In fact, all primitive reflexes have to be lost before voluntary movement can be mastered. Reflex activity usually disappears in normal babies by about eight months of age but may persist in those with neurological damage.
Most of what is known about fetal behaviour has been learned since the introduction of ultrasonography in the 1950s. The fetus exhibits a wide range of behaviours starting with slow flexion and extension of the spine and limbs at around 7.5 weeks’ gestation. The variety of movements increases rapidly over the next 3–4 weeks and many different movement patterns have been described including breathing, truncal rotation, limb flexion/extension, sucking and yawning. As the fetus progresses towards term the movements become more regular and coordinated as a result of increased maturation of the nervous system.
Fetal thumb-sucking can be demonstrated as early as 12–14 weeks’ gestation. Preference for sucking a particular thumb in uterohas been shown to predict head position preference in the newborn and subsequent right or left-handedness. Handedness was thought to be dependent on cerebral lateralisation. However, fetuses prefer one thumb to another from as early as 12 weeks’ gestation, well before the brain has any control over movement. This early sucking behaviour is likely to be under reflex control. Stimulation of the brain is known to influence brain organisation and it is argued that this reflex activity may eventually stimulate the brain to develop ‘handedness’ and subsequent lateralisation of function.
Sensory development in the fetus has been studied mostly in response to sound, and hearing can be shown as early as 23 weeks’ gestation. Fetuses respond with a slowing of the heart rate during maternal speech. There is evidence to suggest that fetuses can differentiate between different speech sounds and show preference for the maternal native language. It may be that experience of speech prenatally begins the process of acquiring language postnatally.
Fetal learning and memory
Fetal learning can be observed in studies of ‘habituation’. Habituation is the decrement in a particular behavioural response that occurs when a new stimulus is presented repeatedly. In an environment of constant sensory stimulation, the ability to ignore meaningless stimuli is essential for the efficient functioning and survival of the fetus. Although very simple, it is one of the most widespread methods of learning and there is good evidence that habituation reflects a healthy nervous system. Habituation of the human fetus has been studied mainly in response to sound and has been shown as early as 23 weeks’ gestation. Stimulation using other sensory modalities such as taste and smell, which are functional at earlier ages, may reveal habituation occurring even earlier in pregnancy. Studies have shown fetal habituation to be predictive of cognitive function in early childhood.
Other aspects of fetal memory have been investigated. Babies whose mothers consistently rested in front of a popular television programme during pregnancy became alert, stopped moving and showed slowing of their heart rate a few days after birth when the theme tune of the programme was played. This behaviour was not shown by babies whose mothers had not watched this programme during pregnancy. This indicates that the fetus is able to learn and remember familiar auditory stimuli, and retain this information over the birth period.
The behaviour of the fetus can be argued to represent the functioning and integrity of its nervous system. By building up a picture of ‘normal behaviour’ of the fetus it is possible to determine the well-being of the fetus. Different fetal behaviour has been observed in pregnancies complicated by maternal smoking or recreational drug abuse, fetal abnormality (for example Down’s Syndrome) and in pregnancies which later spontaneously aborted.
Interest in the causes of cerebral palsy has led many researchers to conclude that most brain damage happens before labour. A recent study from Edinburgh showed that most infants who had neurological symptoms and who died in the newborn period had evidence of brain damage occurring during pregnancy. No obvious cause could be found when the pregnancy was reviewed. Routine ultrasound imaging in pregnancy now occurs throughout the developed world. The development of 4-dimensional ultrasound may eventually provide detailed information about the normality of fetal behaviour. Measurements of the quantity and quality of fetal movement may allow us to determine the severity and extent of any neurological damage if present. The more severely injured the nervous system, the greater the differences in behaviour from a fetus with an intact nervous system. The identification of fetuses exhibiting such behavioural patterns may offer insight into the pathoaetiology of intrauterine brain injury and allow optimal management of these pregnancies.
This article was commissioned and accepted by Professor Neil McIntosh FRCP Edin, Consultant Paediatrician, Department of Child Life & Health, Edinburgh.
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